The present invention relates to a Fabry-Perot fiber optic sensor and, more particularly, to a high finesse Fabry-Perot sensor and tunable laser combination system for measuring small displacements.
Low finesse Fabry-Perot interferometers have a reflectivity of approx. 50% or less. Such interferometers have been used in sensors in order to provide an indication as to the displacement of a gap between two reflective surfaces. For example, U.S. Pat. No. 5,276,501 to McClintock et al. describes a low-finesse Fabry-Perot fiber optic sensor that uses a laser of limited tuning range. The laser itself is tuned via temperature control to operate at two different wavelengths. Notably, reflectivity of the sensor is in the range of 4%, and the readout from the sensor is approximated as a two beam interferometer so that the gap length is a function of wavelength difference and the interference from the additional multiple reflected beams is neglected. While this type of two-beam interferometer approximation may serve the purposes of the McClintock patent, the inventors have accounted for the fact that interference patterns from Fabry-Perot interferometers are not periodic. Consequently, the teachings of the McClintock patent do not apply to Fabry-Perot sensors in the art, especially with respect to the methodology used to perform the calculation of gap length.
Notably, both with respect to the McClintock patent and the other prior references known to the inventors, the range of gaps measurable by prior art laser-based Fabry-Perot sensors were limited in practice by the wavelength and tuning range of the laser. Other errors in such systems resulted from laser instabilities and inability to precisely measure and control laser wavelength.
Given the above limitations and shortcomings of the prior art, a system that is capable of measuring absolute values and monitor small changes in gaps in real time (i.e., at fast scan rates exceeding 2 Hz) would be welcome by the industry. Moreover, a high-finesse system to enable accurate calculations according to true Fabry-Perot equations is needed.